1. Field of the Invention
This invention relates to a projection optical apparatus, and more particularly to an optical apparatus used to project a minute pattern formed on a photomask or a reticle onto a semiconductive wafer.
2. Description of the Prior Art
A projection exposure apparatus using a projection lens can obtain high alignment accuracy and superposition accuracy and greatly contributes to the production of VLSI. Such projection exposure apparatus projects the image of the pattern on the photomask or the reticle onto the wafer having photoresist applied thereto at a predetermined magnification by the projection lens.
Matching accuracy may be mentioned as one of the important performance criteria of such exposure apparatus. An important one of the factors which affect this matching accuracy is the magnification error of the projection optical system. The size of the pattern used in VLSI has a strong tendency toward minuteness and along therewith, the need for improved matching accuracy has become strong. Accordingly, the necessity of keeping the projection magnification at a predetermined value has become very high. At present, the magnification of the projection optical system is at such a degree that the magnification error can be tentatively neglected by adjusting it when the apparatus is installed. However, to sufficiently cope with the tendency of VLSI toward higher density, it is necessary to correct even the magnification error produced when environmental conditions such as the slight fluctuation of the atmospheric pressure in the clean room during the operation of the apparatus change.
A method of mechanically varying the spacing of the projection lens with respect to the object or the image plane or moving the lens elements of the projection lens in the direction of the optic axis to correct the projection magnification of the projection optical system has been popular. However, where highly accurate magnification setting is required, the conventional correcting method which imparts mechanical displacement to the optical elements has been unsuitable because the shift or tilt of the movable portion prevents the optic axis from being properly maintained. That is, the optical system including the object becomes non-coaxial, and this leads to a disadvantage in that a magnification distribution asymmetric with respect to the optic axis occurs on the image plane. Also, to set the magnification accurately so that an error of only 0.05 .mu.m or less occurs on the wafer, it is necessary to control the amount of variation in the optical elements to several .mu.m to 1 .mu.m, inclusive of shift or tilt, and realization of this would involve many difficulties.